Starch is a polymeric carbohydrate material of very high molecular weight. Its monomeric units are termed anhydroglucose units, and the complete hydrolysis of starch yields dextrose. Dextrose in turn is susceptible of isomerization to levulose, either by alkaline or enzyme catalysis. The latter is of increasing importance at the present time because of recent improvements in the conversion of dextrose to levulose by means of enzyme catalysis.
Of all the "sugar" consumed throughout the world, sucrose is by far the most commonly used. It is what is commonly known as table sugar. It is a remarkably stable product and has very good sweetening properties. It is not entirely without shortcomings, however, because at high concentrations it does tend to crystallize and thus adversely affects the texture and appearance of foods in which it is contained. Furthermore, its sweetness is said by some to lack depth and fullness. Dextrose is an alternative, but dextrose lacks the high degree of sweetness which characterizes sucrose. Dextrose is generally rated as being about 60 to 80% as sweet as sucrose and the price at which dextrose is sold is correspondingly lower than that of sucrose. Like sucrose, dextrose tends to crystallize easily.
Levulose, on the other hand, is even sweeter than sucrose, and it does not have the undesirable tendency to crystallize readily.
Unfortunately, levulose does not occur naturally in large quantities and its preparation has heretofore been difficult. Its preparation from sucrose by hydrolysis with hydrochloric acid or with the enzyme invertase has long been known and this hydrolysis produces so-called invert sugar, half of which is levulose and the other half of which is dextrose.
The overall conversion of starch to levulose ordinarily involves three principal, separate steps: a thinning of the starch, followed by saccharification, followed in turn by isomerization. In the first step, an aqueous slurry of starch is heated to gelatinize the starch, and simultaneously, treated with an alpha-amylase or acid, to convert it to an intermediate hydrolysis product having a considerably reduced viscosity with respect to that of the original pasted aqueous starch mixture. Then, in the second step, this intermediate hydrolysis product is saccharified, i.e., converted to dextrose by treatment with a saccharifying enzyme, i.e., a glycoamylase. In the third step, this dextrose product is treated with a glucose isomerase with the resulting formation of a product containing about half dextrose and half levulose, or with a base such as sodium hydroxide to produce a product containing a maximum of about 30% levulose.
Each of the above steps are carried out under different conditions of pH and temperature, so as to optimize the efficiency of each step. Thus, it is necessary to make significant adjustments in these conditions at the conclusion of each step, with the results that the overall efficiency of the process is considerably diminished.
It is accordingly a principal object of the present invention to provide an improved process for the conversion of starch to levulose.
It is another object of the present invention to provide such a process which results in high yields of levulose.
It is another object of the present invention to provide such a process which is characterized also be relatively low temperatures.
It is yet another object of the present invention to provide such a process which can be carried out conveniently and economically in one step.